Formation mechanisms of monolayer pits having characteristic step-edge shapes on annealed GaAs (110) surfaces Akira Ishii a, * , Tsuyoshi Aisaka a , Ji-Won Oh b,c , Masahiro Yoshita b,c , Hidefumi Akiyama b,c , Loren N. Pfeiffer d , Ken W. West d a Department of Applied Mathematics and Physics, Tottori University, Koyama, Tottori 680-8552, Japan b Institute for Solid State Physics, University of Tokyo, Japan c CREST, JST, Kashiwanoha, Kashiwa, Chiba 277-8581, Japan d Bell Laboratories, Lucent Technologies, 600 Mountain Avenue, Murray Hill, NJ 07974, USA Available online 31 July 2004 Abstract An isolated pit and connected pits with one-monolayer depth have characteristic step-edge shapes on a GaAs (110) surface fabricated by a cleaved-edge overgrowth (CEO) method and high temperature growth-interrupt annealing. First principles calculations of Ga and As adatoms have shown that both Ga and As adatoms are most stable at the sites near As of the topmost layer while they are unstable at those near Ga, which is expected to be the microscopic origin for the formation of the fish-shaped pits. D 2004 Elsevier B.V. All rights reserved. PACS: 81.10.Aj; 81.15.Fg; 81.05.Ea Keywords: GaAs; (110); Density functional calculations; Migration 1. Introduction Molecular-beam epitaxial (MBE) growth of GaAs layers on (110) surfaces is a key process in cleaved- edge overgrowth (CEO) [1] for fabricating quantum wires [2] and quantum dots [3] with atomic precision. The optimum MBE growth conditions on (110) surfaces are narrow: a low substrate temperature range (470 – 500 jC) combined with a very high As 4 -vapor overpressure (V/III beam-equivalent pressure ratio of about 70) [1,4]. Thus, they tend to result in a rough surface morphology with large height fluctuation, for example, of about five monolayers (MLs) after 25-ML-thick film growth [5]. On the other hand, it was recently found [5–8] that the rough as-grown (110) surface turns to an automically flat surface if we perform growth-interrupt in situ annealing at 600 jC for 10 min, a process which has been very useful for fabricating high-quality quantum wires [9] and wells [6]. In addition, the growth-interrupt annealing on surface regions with average thickness that deviates from an integer number of MLs leads to the formation of isolated pits or islands with characteristic shapes on an otherwise flat (110) surface. Insufficient coverage makes 1-ML-deep pits shaped like tropical fish pointing towards [001] as shown in an atomic force microscope (AFM) image of Fig. 1(a), while excess coverage results in 2–3-ML-high islands shaped like boats. The fish-shaped pits sometimes appear as a connected row of fish, as shown in Fig. 1(b). The step- edge shapes of the pits and islands are interpreted qualita- tively on the basis of the stability of atomic sites with three or two chemical bonds [7,8]. In this work, we investigate microscopic formation mechanisms of the fish-shaped pits, on the basis of first principles calculations for (110) surface potentials for Ga and As adatoms [10]. Calculated results demonstrate that migration barrier energies for Ga and As are small, which explain long atom-migration length suggested by experi- ments, and that Ga migration is one-dimensional (1D) and As migration is two-dimensional (2D). They also show that 0040-6090/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2004.06.109 * Corresponding author. Tel.: +81-857-315629; fax: +81-857-31- 5629. E-mail address: ishii@damp.tottori-u.ac.jp (A. Ishii). www.elsevier.com/locate/tsf Thin Solid Films 464 – 465 (2004) 38 – 41